part per million
Dimensionlessexact by definition
One part per million is exactly 1e-6 of the whole. It is the standard unit for trace concentrations in water and air, for oscillator frequency stability, and for the tolerance of precision resistors and thermal expansion mismatch. Atmospheric carbon dioxide is about 420 ppm, and a good quartz crystal holds a few ppm over its temperature range. The factor is exact by definition.
Watch out: Parts per million is a ratio and the basis matters. In water chemistry ppm nearly always means milligrams of solute per kilogram of solution, which equals milligrams per litre only because dilute water is close enough to 1 kg per litre. In gas analysis it usually means a volume or mole fraction, written ppmv, and converting that to milligrams per cubic metre needs the molar mass and the temperature and pressure. Two numbers both labelled ppm are not necessarily comparable.
| 1 ppm | 0.000001 — |